Added type_overrides option, which allows you to override types for any
fields in a class. This may later be extended to allow you to override the
return types and argument types of methods as well, if needed. This should
be supplied as an option to a file, and should look like the following:

Support file_overrides option for each html file listed in config files.
This should be a map of file name prefixes to a list of class names. All
classes listed will be output to the corresponding file instead of the
default one.

Start using @HtmlImport and @CustomElementProxy. This should have no effect
on existing applications, other than enabling them to remove some of their
html imports if desired (a dart import alone is now sufficient).

Added deletion_patterns option to the config. This is a list of regex patterns
that match files under the lib/src directory. All matched files will be deleted,
and directories are skipped. This happens before the stubs are generated so they
will not be deleted if you list a folder containing a stub.